'traitorous' originates from English, specifically formed from the noun 'traitor' plus the suffix '-ous', where 'traitor' meant 'one who betrays' and the suffix '-ous' marked an adjective meaning 'full of' or 'having the quality of'.

'traitor' changed from Old French 'traitour' (from Medieval Latin 'traditor'), which came from Latin 'traditor' (from the verb 'tradere' meaning 'to hand over'); it entered Middle English and eventually became the modern English 'traitor', and then 'traitorous' and 'traitorously' developed in English by suffixation.

Initially, the root implied 'one who hands over' (literal handing over); over time it evolved to mean 'one who betrays or delivers another to an enemy', and the adjective/adverb forms came to mean 'showing betrayal' or 'in a betraying manner'.